Transistors such as metal oxide semiconductor field effect transistors (MOSFETs), or simply field effect transistors (FETs), or MOS transistors are important building blocks of the vast majority of semiconductor integrated circuits (ICs). An FET includes source and drain regions between which a current can flow through a channel under the influence of a bias applied to a gate electrode that overlies the channel. Some semiconductor ICs, such as high performance microprocessors, can include millions of FETs. For such ICs, decreasing transistor size and thus increasing transistor density has traditionally been a high priority in the semiconductor manufacturing industry. Generally, transistor performance is maintained even as the transistor size decreases.
Often, transistors have metal gate structures utilizing tungsten as a metal fill portion in conjunction with a titanium nitride (TiN) P-type work function metal layer, and/or a tantalum carbide (TaC) or a titanium carbide (TiC) N-type work function metal layer. Usually, the tungsten metal fill portion is used as a conductive metal fill to offset the relatively higher resistance of the work function metal layer(s), particularly TaC or TiC for N-type work function metal, to lower the overall resistance of the metal gate structure. The use of different materials for P-type work function metal layers and N-type work function metal layers complicates manufacturing of P-channel FET (PFET) and N-channel FET (NFET) devices.
Furthermore, oxidation can affect gate layers by increasing the threshold voltage to attain effective work function, particularly for NFET devices. A relatively thin layer of a work function metal, which is usually desired to decrease transistor size, may be susceptible to oxidation, which can impede effective work function at targeted threshold voltages.
Accordingly, it is desirable to provide integrated circuits and methods for fabricating such integrated circuits that use a single metal for both P-type and N-type work function metal layers. Moreover, it is desirable to provide integrated circuits and methods for fabricating such integrated circuits that use a work function metal layer having sufficient thickness so as to not adversely affect work function due to oxidation. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and the background of the invention.